Understanding the Photoluminescence Quenching of Liquid Exfoliated WS2 Monolayers

Monolayer transition metal dichalcogenides (TMDs) are being investigated as active materials in optoelectronic devices due to their strong excitonic effects. While mechanical exfoliation (ME) of monolayer TMDs is limited to small areas, these materials can also be exfoliated from their parent layered materials via high-volume liquid phase exfoliation (LPE). However, it is currently considered that LPE-synthesized materials show poor optoelectronic performance compared to ME materials, such as poor photoluminescence quantum efficiencies (PLQEs). Here we evaluate the photophysical properties of monolayer-enriched LPE WS2 dispersions via steady-state and time-resolved optical spectroscopy and benchmark these materials against untreated and chemically treated ME WS2 monolayers. We show that the LPE materials show features of high-quality semiconducting materials such as very small Stokes shift, smaller photoluminescence line widths, and longer exciton lifetimes than ME WS2. We reveal that the energy transfer between the direct-gap monolayers and in-direct gap few-layers in LPE WS2 dispersions is a major reason for their quenched PL. Our results suggest that LPE TMDs are not inherently highly defective and could have a high potential for optoelectronic device applications if improved strategies to purify the LPE materials and reduce aggregation could be implemented.


■ INTRODUCTION
The study of transition metal dichalcogenides (TMDs) has become a vibrant area in nanomaterial science. 1,2 Exfoliated TMDs have been widely used in the field of optoelectronics due to their excellent light absorptivity and semiconducting performance. 3−6 To utilize TMD materials, achieving more scalable techniques is critically important, and considerable effort has been devoted to the development of cost-effective mass production methods. 7,8 Although mechanical exfoliation (ME) produces the highest quality materials, its application is limited by extremely low and uncontrollable yield. 9,10 In contrast, liquid exfoliation yields atomically thin TMD flakes in a liquid medium in large quantities at moderate cost. 11,12 The simplest way to produce nanosheets suspended in liquid is termed liquid phase exfoliation (LPE) which relies on immersing the bulk materials into suitable solvents or aqueous surfactant solution and applying high energy, e.g., sonication, to achieve exfoliation accompanied by tearing. 13,14 When appropriately chosen, the solvent or surfactant suppresses reaggregation in the liquid. 15 This approach is widely applicable to a range of materials and takes advantage of well-established print production processes for device fabrication. 16,17 Despite their potential advantages, LPE TMD nanosheets have been rarely used in optoelectronic devices, due to their lower optoelectronic quality in comparison to ME TMDs. A key metric for the optoelectronic quality of a semiconductor is its photoluminescence quantum efficiency (PLQE). 18 Defects and impurities, as well as chemical and structural inhomogeneity, etc., can lead to nonradiative recombination and hence suppress PLQEs. While it is known that LPE TMDs show poor PLQE, what causes this is unclear. This lack of photophysical understanding is contrasted with great progress in the development of the liquid phase exfoliation methodology and subsequent size selection. 19,20 Largely defect-free monolayer enriched TMD dispersions with narrow line width photoluminescence (PL), which are similar to that of ME TMDs, have been demonstrated. 21,22 However, the PLQE of these LPE TMD dispersions remains low and exciton dynamics of TMD dispersions are only little explored. 23 In particular, most reports focus on ensembles with low monolayer content produced from either LPE or colloidal synthesis. 24−27 Also, to the best of our knowledge, no reports are making a direct comparison of LPE TMDs vs high-quality ME TMDs, which makes evaluation of the inherent properties of LPE TMDs difficult.
Recently, we showed that chemical treatment of mechanically exfoliated WS 2 with bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) allows for greatly suppressed nonradiative decay and trap-free PL emission with intensity over 100 times the untreated monolayers. 28 In this work, using these high-quality ME systems as a benchmark, we evaluate the quality of monolayer-enriched WS 2 dispersion produced from LPE. We conduct a systematic study and comparison of the optical and photophysical properties of different LPE WS 2 dispersions with ME WS 2 monolayer samples. We find that the LPE WS 2 dispersions can achieve PL with narrow line width and almost no Stoke shift compared to their absorption spectra. In addition, we use ultrafast pump−probe spectroscopy to study the exciton dynamics following photoexcitation in these WS 2 samples. We reveal that the energy transfer between monolayers and few-layers in LPE WS 2 dispersion samples is a major factor in quenching the PL and reducing PLQE yields, suggesting that LPE TMDs can offer very high optoelectronic performance if better size selection can be achieved. ■ METHODS Materials. WS 2 powder (99%, 2 μm, Sigma-Aldrich), surfactant sodium cholate hydrate (SC, ≥99%), and bis-(trifluoromethane)sulfonimide lithium salt (Li-TFSI) are purchased from Sigma-Aldrich and used without purification. The bulk synthetic WS 2 crystal is purchased from 2D Semiconductors. The mechanically exfoliated monolayer WS 2 is prepared according to the reported gold-mediated exfoliation method to ensure relatively large monolayers. 29 Liquid Exfoliation Process. WS 2 powder (30 g/L) and SC (8 g/L) are added to a glass bottle with 80 mL of deionized water, and the dispersion is transferred to a stainless-steel beaker for sonication. The beaker is placed in a cooling water bath with a temperature of 5°C (maintained through a chiller). An ultrasonic replaceable tip is positioned in the dispersion ∼2 cm from the bottom, and the mixture is sonicated for 1 h with 60% amplitude (pulse 8 s on and 2 s off ratio), using a Sonics Vibracell VCX 500, equipped with a threaded probe. The metal beaker is covered with aluminum foil during the sonication process. After the sonication, the dispersion is centrifuged at 6000 rpm for 1.5 h at 8°C in a Hettich Mikro 220R centrifuge, equipped with a 1016 fixedangle rotor. The participants are removed afterward and 2 g/L SC solution is added to the dispersion to reach 80 mL, followed by another tip sonication with the same amplitude at 5°C for 5.5 h. The first sonication step serves the purpose of removing impurities in the WS 2 powder. After the second sonication, the dispersion is transferred to centrifuge tubes for size selection by liquid cascade centrifugation. 21 First, the dispersions are centrifuged with relative centrifugal force (RCF) of 5k g for 1 h at 8°C. Supernatant and sediment are separated through manual pipetting. The supernatant is collected and centrifuged at the same speed for 2 h at 8°C to remove large/thick sheets as completely as possible. Then the supernatant is centrifuged with RCF of 10k g for 2 h at 8°C. The sediment is collected and dispersed in 0.1 g/L SC solution (∼ 2 mL), which is referred to 5−10k g WS 2 /H 2 O sample. The supernatant is transferred to new centrifuge tubes for further centrifugation with RCF of 30k g for 2 h at 8°C. In the end, the supernatant is discarded, while the sediment is collected and dispersed in 0.1 g/L SC solution (∼ 2 mL), which is referred to 10−30k g WS 2 /H 2 O sample. To transfer the 10− 30k g WS 2 /H 2 O sample from water to IPA, the dispersion is centrifuged with RCF 30k g for 1.5 h to pellet out the nanosheets as sediment and decant the water supernatant. The sediment is redispersed in IPA through 5 min bath sonication. Then the dispersion is centrifuged with RCF of 2k g for 20 min to remove the majority of aggregates. The supernatant is collected as a 10−30k g WS 2 /IPA sample.
Chemical Treatment. The chemical treatment with Li-TFSI (0.02 M in methanol) is carried out in the ambient atmosphere. The chemical treatments are achieved by immersing the samples into concentrated solutions of the investigated chemicals for 40 min and blow-drying with a nitrogen gun afterward.

■ RESULTS AND DISCUSSION
The LPE WS 2 dispersion samples used in this study were prepared as shown in Figure 1. The liquid-suspended WS 2 nanosheets are generated with the aid of dip sonication and stabilized against reaggregation by the surfactant sodium Relative centrifugal forces of (RCFs) 5k g, 10k g, and 30k g are used. The supernatant after each step is transferred to another centrifugation at higher centrifugal acceleration, while the sediments are collected. (c) Schematic of the dispersion solvent exchange, transferring from H 2 O to IPA. 30k g RCF is used to spin-down the WS 2 nanosheets as a pellet allowing for solvent exchange. 2k g RCF is used to remove aggregated nanosheets as sediments.
cholate in water. A size selection process is followed since the as-produced dispersion is highly polydisperse displaying a low monolayer content. Size selection is achieved by liquid cascade centrifugation (LCC) with subsequently increasing rotational speeds. 30 Heavier and multilayer nanosheets are removed in each step of the LCC process, resulting in more and more monolayer-enriched supernatants. Two size-selected nanosheet distributions are collected as sediments after 10k g and 30k g centrifugation, hereafter labeled as 5−10k g WS 2 /H 2 O sample and 10−30k g WS 2 /H 2 O sample, respectively. Isopropyl alcohol (IPA) is also known to give stable dispersions; however, monolayer enrichment has not yet been demonstrated. Hence, a 10−30k g WS 2 /IPA sample is also prepared in comparison by replacing the water/surfactant in the 10−30k g WS 2 /H 2 O sample with IPA through a centrifugation procedure. The monolayers obtained in this way are around 50 nm as characterized by transmission electron microscopy (TEM) ( Figure S1a), which is similar to what was obtained from previous work. 21 Since mechanical exfoliation renders high-quality TMD monolayers, ME WS 2 samples are also prepared as a reference to LPE WS 2 samples. Large monolayer WS 2 samples (∼200 μm) prepared on quartz substrates with mechanical exfoliation are identified by optical microscopy ( Figure S1b). As shown in Figure S1c, both LPE and ME WS 2 samples on Si/SiO 2 substrates are characterized by Raman spectroscopy (excitation wavelength 532 nm), confirming the monolayer with characteristic Raman modes of monolayer WS 2 (e.g., the 2LA(M) at 354 cm −1 ). 31,32 The optical absorption properties of LPE 5−10k g WS 2 / H 2 O, 10−30k g WS 2 /H 2 O, and 10−30k g WS 2 /IPA dispersions are characterized via UV−vis extinction spectroscopy. The optical properties are summarized in Table 1. The spectra depicted in Figure 2a and Figure S2a are normalized to the local minimum at 290 nm since the extinction coefficient at 290 nm is independent of nanosheet thickness and length. 33 The absorption spectra are dominated by excitonic features. The A-exciton (E A ML ) for all LPE WS 2 dispersions is analyzed in more detail using the second derivative of the extinction spectra (Figure 2b and Figure S2b). Due to the previously identified exponential blueshift of the A-exciton with decreasing layer number, two components are visible in the second derivative attributed to the A-exciton of the monolayer (E A ML ) and the unresolvable sum of few-layers (E A FL ). 21 35 Since the contributions to the A-exciton absorbance of monolayer and few-layer WS 2 nanosheets are differentiated, the monolayer content is estimated from the second derivative of the A-exciton absorbance peak according to the previously reported method (described in Supporting Information). 21 There is a clear increasing monolayer volume fraction (V f ) in water dispersions with increasing RCF, which is 17% for the 5−10k g WS 2 /H 2 O sample and 78% for the 10−30k g WS 2 / H 2 O sample, respectively. On the other hand, the 10−30k g  ) and fewlayer A-exciton (E A WS 2 /IPA sample shows a moderate V f at around 35%, suggesting that some aggregation occurred during the solvent transfer. In the following, we focus on the optical and photophysical properties of the monolayer-enriched LPE 10− 30k g WS 2 /H 2 O dispersion and 10−30k g WS 2 /IPA dispersion samples. To evaluate the quality of LPE WS 2 samples, we start by comparing the steady-state PL profiles of LPE and ME WS 2 samples. The PL of ME WS 2 monolayers is measured with a confocal PL setup, while the LPE samples are measured as dispersions. As shown in Figure 2c, the PL position of ME untreated monolayer WS 2 sample (E A ML , PL) is around 1.981 eV with a full width at half-maximum (fwhm) value of around 44 meV, indicating the emission mainly stems from a dominating contribution of trions in the sample. 9 After Li-TFSI treatment, the PL of ME monolayer WS 2 is greatly enhanced and the peak position blue-shifts accompanied by a more uniform emission profile due to the suppression of trions and defects, as shown in scatter plots of the peak PL counts versus emission peak position acquired from PL spatial maps ( Figure S3). In addition, the Li-TFSI-treated WS 2 sample exhibits a narrower fwhm of around 10 meV. The PL Stokes shift of the ME Li-TFSI treated WS 2 sample is primarily related to strain. 36 This is in good agreement with our previous work showing that Li-TFSI treatment can minimize trap and trion states resulting in intrinsic monolayer properties. 10,28 The PL positions of both the LPE 10−30k g WS 2 /H 2 O dispersion and the 10−30k g WS 2 /IPA dispersion coincide with that of the monolayer A-exciton absorbance with almost no Stokes shift, suggesting a high optical quality of the samples with near intrinsic properties, Table 1. However, the monolayer enriched LPE WS 2 dispersions show extremely low PLQE, less than 0.1% as it is too low to determine accurately with our setup. Turning to the fwhm of the PL from the LPE samples, we find that it is around 19 meV for both LPE WS 2 samples, which is broader than that of the ME Li-TFSI treated WS 2 samples (10 meV) but narrower than the untreated ME WS 2 samples (44 meV). This may be ascribed to polydispersity-induced defectrelated broadening of the exciton resonances. 37 These results hint at the puzzling nature of this system, while the lack of Stokes shift and relatively narrow PL line width from the LPE samples would suggest a reasonably high material quality with much fewer defects on the basal planes compared to untreated ME WS 2 samples, yet the sample shows very poor PLQE. We note that there is a long PL tail below the bandgap of WS 2 in the LPE 10−30k g WS 2 /IPA dispersion that is attributed to the larger portion of a few layers (i.e., nonmonolayers) caused by aggregation. The aggregated nonmonolayer materials would not be expected to show high PL as they would not be directgap. Hence the presence of PL from them would indicate that they play a larger role than expected in these systems.
To investigate the reason for the low PLQE of LPE WS 2 dispersions, we conducted ultrafast pump−probe spectroscopy to explore the exciton dynamics of the LPE WS 2 dispersions and the ME WS 2 monolayer samples. Upon excitation, the state filling of the A-exciton leads to a reduction in the groundstate absorption, which is referred to as the A-exciton groundstate bleach (GSB). We record the differential transmission (ΔT/T) of a white light probe beam as a function of time after photoexcitation by a pulsed laser. The chirp is determined by measuring a blank sample ( Figure S4a). The 2D maps of the LPE WS 2 dispersions and the ME WS 2 monolayer samples  Figure S4b−e. The full pump−probe spectra of the LPE 10−30k g WS 2 /H 2 O dispersion, the WS 2 /IPA dispersion, the ME untreated WS 2 , and the Li-TFSI treated WS 2 samples excited at around the Aexciton resonance 610 nm (2.033 eV) with 2.63 nJ/pulse are shown in Figure S5. Dynamic screening of Coulomb interaction gives rise to either a comparatively small red-shift or blue-shift of the A-exciton resonance depending on the exciton density. 38,39 As shown in Figure 3a,b and summarized in Table 1, the monolayer A-exciton GSB maximum (E A ML , GSB) is located at 611 nm (2.029 eV), 614 nm (2.019 eV), and 616 nm (2.013 eV) for the LPE WS 2 /H 2 O dispersion, the WS 2 /IPA dispersion, and the ME Li-TFSI treated WS 2 sample, respectively. This coincides with E A ML (PL), confirming that the PL of LPE WS 2 dispersions and ME Li-TFSI treated WS 2 sample stems from neutral exciton emission, while E A ML (GSB) is detected at 618 nm (2.006 eV) for the ME untreated WS 2 monolayer, which is red-shifted compared to E A ML (PL). This is in good agreement with our interpretation that the PL of the ME untreated WS 2 monolayer is dominated by trion emission. In contrast to the ME monolayers, LPE samples also display a positive feature at around 650−690 nm which we assign to the few-layer A-exciton GSB (E A FL , GSB). The few-layer signal is more prominent in the LPE WS 2 /IPA sample than that in the 5−10k g and 10−30k g WS 2 /H 2 O samples ( Figures S5 and  S6), even though the monolayer content in the WS 2 /IPA dispersion is larger than that in the 5−10k g sample. This suggests that this feature is a signature of aggregated nanosheets that is increased in content during the solvent exchange process (detailed discussion in Supporting Information, Table S1, on pages S8−S9).
The normalized kinetics taken at the A ML -exciton GSB and A FL -exciton GSB are shown in Figure 3c, and the averaged decay lifetimes (⟨τ⟩) are summarized in Table 1 while the fitting results are exhibited in Table S2. The time zero is defined as the maximum intensity of the A ML GSB after chirp correction. It turns out that it is also the time that A FL GSB starts to rise. For the ME untreated monolayer WS 2 sample, photogenerated excitons decay primarily through nonradiative recombination on a few picoseconds time scale. 40,41 After Li-TFSI treatment, ⟨τ⟩ increases to tens of picoseconds, which is ascribed to radiative recombination. Surprisingly, the A MLexciton GSB for the LPE 10−30k g WS 2 /H 2 O dispersions exhibits longer lifetimes than the high-quality Li-TFSI treated ME samples, again suggesting a high material quality and slower nonradiative recombination.
For both the LPE 10−30k g WS 2 /H 2 O and WS 2 /IPA dispersions, A FL -exciton GSB rises simultaneously while A MLexciton GSB goes through a fast decay at a time scale of picoseconds. The decay of the A ML -exciton GSB and the initial rise and later decay of the A FL -exciton GSB are fitted simultaneously with the constraint that the initial decay constant of the A ML -exciton GSB and rise of the A FL -exciton GSB are the same. Satisfactory fits are obtained with three exponential decays and an additional initial exponential rise for the A FL -exciton GSB decays. Importantly, the concomitant rise and decay of mono and multilayer signals indicate that there is energy transfer between monolayers and multilayers in LPE WS 2 dispersions. This energy transfer from the high-quality direct-gap monolayers to the in-direct gap multilayers may be causing the loss of PL, as the in-direct gap multilayers would be expected to have poor PLQE.
To test our hypothesis, we conduct further pump−probe measurements by exciting the LPE 10−30k g WS 2 /IPA dispersion sample with an energy below the WS 2 bandgap, at 650 nm, to directly excite the multilayer components and observe the exciton decay. Since the position of A FL -exciton GSB red-shifts with increasing layer number, the broad positive features shown in Figure 4a are assigned to the A FL -exciton GSB, confirming the existence of lower energy band states due to multilayer nanosheets. The 2D map after chirp correction is shown in Figure S4f. It is worth noting that the positive feature at around 620 nm is actually broader than it appears, due to the bandgap renormalization, and the energy transfer can happen from a few layers to more aggregated multilayers which have a broader A-exciton GSB. 42 As shown in Figure 4b and summarized in Table S3, simultaneous with the fast A FLexciton (∼620 nm) decay is the growth of more aggregated A FL -exciton (∼660 nm) GSB. The further extended ⟨τ⟩ at A FLexciton GSB (∼660 nm) supports our hypothesis that there is energy transfer between the individual sheets in the LPE WS 2 dispersion. In this scenario, we propose that the optical quality and specifically the PLQE of the LPE WS 2 can potentially be improved by reducing the energy transfer by introducing a coating on the nanosheets, for example, through chemical To gain further insight into the exciton dynamics of the LPE WS 2 samples, we also analyze the photon energy dependence and pump fluence dependence of the A-exciton GSB decay feature (Figures S7−S10; Tables S4−S7). A subpicosecond decay component in the excited-state dynamics of WS 2 emerges for incident photon energies above the A-exciton resonance. This originates from a nonequilibrium population of charge carriers that form excitons as they cool and is dependent on the photon energy. 42 Nevertheless, the exciton decays for LPE WS 2 samples and ME untreated WS 2 samples are largely independent of the pump fluence ( Figures S7 and  S8). The fluence-independent nature of the recombination indicates that it is linked to defect-assisted decay, 43 while the ⟨τ⟩ at A ML -exciton GSB for ME Li-TFSI treated WS 2 samples shortens with the increase of pump fluence due to the enhanced EEA process. This suggests that there are also other reasons for the low PLQE of LPE WS 2 dispersions besides energy transfer, such as edge effects related to the small lateral dimensions of the monolayers in LPE dispersions. Hence the liquid exfoliation process needs to be further improved to produce samples suitable for practical optoelectronic application. 33 In conclusion, monolayer-enriched WS 2 dispersions produced by liquid phase exfoliation (LPE) combined with size selection and WS 2 monolayers by mechanical exfoliation (ME) were prepared, and the photophysical properties of the samples were compared with steady-state and time-resolved optical spectroscopy. Our results reveal that even though LPE monolayer-enriched WS 2 dispersions can exhibit pristine-like excitonic features, such as narrow linewidth PL and minimal Stoke shift, the photoluminescence quantum efficiency (PLQE) is very low (<0.1%). Detailed analysis of the exciton dynamics of LPE WS 2 dispersions through pump−probe spectroscopy suggests that this poor PLQE results from energy transfer between individual monolayer and multilayer nanosheets in the LPE WS 2 dispersions. The energy transferred to the in-direct gap multilayers then leads to nonradiative recombination. Other factors increasing nonradiative recombination and hence lowering PLQE may include the small lateral sheet size of the LPE samples. Our results suggest that LPE TMDs are not inherently highly defective and could have a high potential for optoelectronic device applications. Instead, aggregation and energy transfer from monolayer to aggregated materials are the cause of the poor PLQE. We, therefore, propose that improved strategies to purify the LPE materials and reduce aggregation could significantly improve the PLQE of these materials. Thicker coatings on the sheets, such as polymers as additional stabilizers, could also potentially reduce the energy transfer phenomenon.